Two dimensional self-avoiding walk with hydrogen-like bonding: Phase diagram and critical behaviour

The phase diagram for a two-dimensional self-avoiding walk model on the square lattice incorporating attractive short-ranged interactions between parallel sections of walk is derived using numerical transfer matrix techniques. The model displays a collapse transition. In contrast to the standard $\theta$-point model, the transition is first order. The phase diagram in the full fugacity-temperature plane displays an additional transition line, when compared to the $\theta$-point model, as well as a critical transition at finite temperature in the hamiltonian walk limit.Comment: 22 pages, 13 figures. To appear in Journal of Physics

Two-Dimensional Polymers with Random Short-Range Interactions

We use complete enumeration and Monte Carlo techniques to study two-dimensional self-avoiding polymer chains with quenched ``charges'' $\pm 1$. The interaction of charges at neighboring lattice sites is described by $q_i q_j$. We find that a polymer undergoes a collapse transition at a temperature $T_{\theta}$, which decreases with increasing imbalance between charges. At the transition point, the dependence of the radius of gyration of the polymer on the number of monomers is characterized by an exponent $\nu_{\theta} = 0.60 \pm 0.02$, which is slightly larger than the similar exponent for homopolymers. We find no evidence of freezing at low temperatures.Comment: 4 two-column pages, 6 eps figures, RevTex, Submitted to Phys. Rev.

Mesure de la conductivité thermique et détermination de l'orientation des molécules a l'interface nématique isotrope de MBBA

We have investigated the thermal properties of a nematic liquid crystal (MBBA). We have determined the temperature dependence of the anisotropic heat conductivity. We have also studied the interface between the nematic and isotropic phases. The boundary condition is oblique (conical condition). Several applications are discussed (thermal hysteresis, disclination lines in the plane of the interface, possible heat convection effects).Nous avons étudié les propriétés d'un film de cristal liquide nématique (MBBA) en présence d'un gradient de température. Nous avons, d'une part, mesuré la variation, en fonction de la température, des conductivités thermiques k∥ et k〉. Nous avons, d'autre part, déterminé l'orientation des molécules à l'interface nématique-isotrope, par des observations optiques. Nos expériences montrent que les molécules de MBBA sont inclinées par rapport à l'interface (raccordement conique). Quelques applications sont présentées

Equilibrum and surface rheology of monolayers of insoluble polycations with side chains

We have studied monolayers of poly(n-tetradecyl 4-vinylpyridinium-co-4- vinylpyridine) bromide with different degrees of quaternization at the air-water interface. The isotherms (surface pressure vs area) present several phase transitions: at low monolayer coverage, there is a phase transition over a characteristic area that increases on increasing the quaternization degree. This behavior can be rationalized in terms of a mean-field theory of 2D semiflexible polymeric chains and could be an indication of a disorder-order transition from a 2D isotropic liquid (IL) at low surface concentration to a 2D nematic phase (N) at higher concentrations. Low-frequency oscillatory strain experiments show that at low surface coverage the monolayers exhibit highly nonlinear behavior, even for low strain amplitude, whereas at higher surface coverage the response is linear for strains higher than 20%. In addition, stress relaxation experiments show a minimum in the characteristic times that coincide with the transition area. These unexpected results at low surface coverage might be characteristic of the system or related to the fact that the oscillatory experiments do not strictly correspond to constant surface-coverage conditions. However, they are in agreement with high-frequency viscoelasticity, obtained by surface quasielastic light scattering, that shows that the dilational viscosity is higher at low surface concentration than for concentrations beyond the surface phase transition. At higher coverage, there is a second phase transition, after which the isotherms present hysteresis, which is not observed below. Ellipsometry indicates that, after this transition, the monolayer thicken, which may be related to 3D growth into a multilayer. © 2009 American Chemical Society.info:eu-repo/semantics/publishe